The present invention relates generally to a system combining a synchronization function of a network element with a network interface function.
A Digital Loop Carrier (DLC) System is a component of a communications network that bundles a number of individual phone line signals into a single multiplexed digital signal for local traffic between a telephone company central office and a business complex, subscriber, or other outlying service area. Digital loop carriers can carry traffic for regular phone calls (Plain Old Telephone Service or POTS) and Integrated Services Digital Network (ISDN) service. More recently, approaches have been developed for using DLCs to handle the higher bandwidth of Digital Subscriber Loop (DSL) service. Such approaches are often referred to as Broadband Loop Carrier (BLC) systems. FIG. 1 generally shows such a communications network.
In order for the BLC system to function, it must remain synchronous with respect to network data signal timing. Further, each network element in the network requires a stable clock signal synchronous to the network data signals in order to maintain proper signal timing. For this reason, network elements such as BLC systems often extract synchronization signals either from a signal source or from a dedicated timing source such as a Building Integrated Timing Source (BITS) generator.
The signal sources, or data signal inputs, to a network element can be in the form of optical signals, such as OCN (Optical Carrier level N), or electrical signals, such as T1. Both types of signal carry data in the form of data bits and synchronization information in the form of signal transitions. Network elements typically extract both data and clock information from their inputs. The synchronization information typically requires local processing in the form of filtering in order to make it suitable for synchronization with the local network element. Since the signal has passed over an imperfect transmission network, periodic or random variations in the period of the digital signal may occur. A phase-locked loop (PLL) and stable local oscillator perform this function. The filter removes short and long-term variations in frequency of the recovered synchronization signals. The local stabilized oscillator also allows the network element to remain synchronized to the network if a connection to the network is temporarily lost, within tolerances specified in BellCore standard GR-1244-CORE.
In network elements serving a large number of end users, reliability of the synchronization system is typically designed to be more robust than that found in smaller systems. In order to achieve the required level of reliability, these systems typically employ a form of redundant synchronization hardware along with redundant synchronization inputs from the network. Along with a control element, these redundancy schemes are able to automatically recover either from synchronization input failures or hardware failures by switching to the backup. The requirements for various levels of redundancy are specified in BellCore documents GR-1244-CORE, GR-303-CORE, and GR-253-CORE.
In a typical network element, dedicated hardware modules in the form of circuit boards perform the synchronization. These modules receive synchronization signals from the communications network and provide the network element with the necessary synchronization signals to provide whatever service it is intended to provide. Since synchronization uses dedicated equipment, the extra equipment occupies space in the system cabinet or shelf, which could otherwise be used to increase functionality. It also adds cost, mechanical complexity, and increased points of potential failure, thereby reducing product reliability.
Therefore, it is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.